yeah, you can do it.. but you have to realize there will be a quality loss in manual KVM's.. and you'll have to make sure both KVM boxes are set to the correct positions when switching between computers.
You will also need an additional KVM cable...
If you have a better KVM (like a Belkin Omniview) they are made to allow "daisy chaining" of KVMs, so I could theoretically have upto 16(maybe more) computers connected using 4 port KVM switches. Since the omniview series is an electronic KVM there should be a minimal amount of quality loss when compared to manual KVM switches.
Personally, if you're using a manual KVM and video resolutions 1024x768 or higher I think you'd be best of buying a 4 port Omniview from ebay... I got mine for ~$10 (like new comdition)and about $10 for an additional cable... had to scrounge for a power supply, but got one used for ~$1 from a local computer shop.
I have the older omniview and I seem to have problems using the mousewheel in linux (when switching back and forth) so I'd recomend one of the newer omniviews that specifically support mousewheel emulation.
The benefit of an electronic KVM is that you don't have video quality loss(which is especially noticable at higher resolutions and with more computers) and you can switch between computers by using keyboard shortcuts instead of a switch located on the KVM.
This is the model I have,
this is the one I'd recomend if using linux.